In the technical field of exciting a plasma to electron cyclotron resonance (hereinafter abbreviated to “ECR”), resonance is obtained when the frequency of gyration of an electron in a static or quasi-static magnetic field is equal to the frequency of the applied accelerating electric field. This resonance is obtained for a magnetic field B at an excitation frequency f which is related to B by the following relationship:B=2πmf/e  (1)wherein m and e are the mass and the charge of an electron.
When exciting a plasma at electron cyclotron resonance frequency electrons revolve in phase with the electric field and continuously gain energy from the external excitation source where the ECR condition (1) is met such as to reach the threshold energy necessary for dissociating or ionizing the gas. To satisfy this condition, it is necessary firstly that the electron remains trapped in the magnetic field lines, i.e. that its radius of gyration is small enough with respect to the static magnetic field gradient for the electron to see a substantially constant magnetic field during its gyration, and secondly that the frequency of gyration remains large relative to the frequency of collision between electrons and neutral elements such as atoms and/or molecules. In other words, the best conditions for exciting a plasma to electron cyclotron resonance are expected to be obtained when simultaneously the gas pressure is relatively low and the excitation frequency f is high, which also means that the magnetic field intensity B must be high.
A major difficulty with conventional divergent ECR is that it is not possible to produce a plasma whose density is substantially uniform over a large area. This means that it cannot be used, for example, to deposit a substantially uniform layer of material on a work surface of large size. To overcome this problem, a technique has been developed which is known as distributed electron cyclotron resonance (DECR), which employs an apparatus in which a plurality of plasma excitation devices is formed into a network, with the devices collectively generating a plasma whose density is substantially uniform at the work surface. The individual plasma excitation devices are each constituted by a wire applicator of microwave energy, having one end connected to a source for producing microwave energy and having an opposite end fitted with at least one magnetic dipole for creating at least one surface having a magnetic field that is constant and of an intensity corresponding to electron cyclotron resonance. The dipole is mounted at the end of the microwave applicator in such a manner as to ensure that electrons accelerated to electron cyclotron resonance oscillate between the poles so as to create a plasma diffusion zone situated on the side of the dipole that is remote from the end of the applicator. The individual excitation devices are distributed relative to one another and in proximity with the work surface so as to create together a plasma that is uniform for the work surface.
Such a DECR apparatus is described in U.S. Pat. No. 6,407,359 (corresponding to EP-1075168), and more detailed discussion of the apparatus described therein is given below, with reference to drawings. As is apparent from those drawings, excitation devices, as viewed from the substrate, take the form of a generally rectangular array, by which we include also the particular case where the rectangle is a square, and such an apparatus is therefore sometimes referred to as matrix DECR (MDECR) apparatus. It is to be understood, however, that the present invention could also be applied to a DECR apparatus where the excitation devices were arranged in a non-rectangular two-dimensional network, for example in a hexagonal network or where there are two parallel lines of devices, with the devices in one line being offset with respect to one another. An example of a hexagonal array is given in “Determination of the EEDF by Langmuir probe diagnostic in a plasma excited at ECR above a multipolar magnetic field”, T. Lagarde, Y. Arnal, A. Lacoste, J. Pelletier, Plasma Sources Sci. Technol. 10, 181-190, 2001. The devices could also be disposed as a circular, part-circular, or near-circular array. It should be noted that in some work done by the present inventors, depositions have been carried out with a central plasma excitation device being surrounded by three or six devices, the surrounding devices having the polarity of their magnets being oppositely disposed to the magnet of the central device and being arranged in a triangular or hexagonal array respectively.
Furthermore, the invention can be applied to a DECR apparatus which is not of an MDECR type. Thus, for example, it is applicable to a DECR reactor which, historically, preceded the MDECR type, and which has a cylindrical shape and uses long antennas and magnets that extend from the top to the bottom of the cylinder. Such arrangement is described in “Microwave Excited Plasmas” by Michel Moisan and Jacques Pelletier, Elsevier, 1992, and would be suitable for homogeneously coating a cylindrical substrate such as a tube or an object which is characterized by a dimension (length, radius) which is small as compared to the plasma ambipolar mean free path (See above reference, Appendix 9.1 page 269-271). This object can have a flat surface lying in the central part of the plasma and oriented perpendicular to the axis of the cylinder.
Unlike most divergent ECR reactors, DECR reactors employ only a single chamber, which serves both as a plasma chamber and a deposition chamber. This allows direct decomposition of the precursor gas by the plasma without the use of an additional and specific plasma gas. The substrate is not located in the plasma or directly in the intense magnetic field, which prevents the unintentional bombardment of the growing film by hot electrons and ions. The plasma is generated in each of the MW-ECR zones, in close proximity to the antennas, and in particular close to the magnet generating the ECR zone.
The creation of a network of MW-ECR antennas has the benefit of allowing the plasma region to be expanded, and of the generation of a homogeneous flow of species towards the substrate. FIG. 3 of the accompanying drawings shows a plasma created by four antennas.
In DECR the use of a plasma gas different from the film precursor gas is not essential and a film precursor gas can be used alone, without an additional plasma gas. In such cases the film precursor gas decomposes in the vicinity of the antennas and diffuses toward the substrate to deposit and form the film. During this “travel” secondary reactions may occur between the plasma-generated species and un-dissociated gas. For instance, where the precursor gas is SiH4, hydrogen radicals generated by the decomposition of SiH4 can react with un-dissociated SiH4 to form SiH3 radicals, which are believed to be the most important radicals required for the deposition of high quality films.
However, although not essential, a plasma gas can be used in DECR, in addition to the film precursor gas. Examples of such plasma gases are H2, Ar and He. It is desirable that these plasma gases are excited or decomposed by the plasma prior to reacting with the film precursor gas. Such a requirement is complex to achieve with the conventional way of injecting the gases into a DECR reactor, particularly if the production of a large area plasma is required for the deposition of a homogeneous film on a very large area.